1. Field of the Invention
The present invention relates to infrared detection systems. More specifically, the present invention relates to systems and methods for providing automatic gain and level control in infrared detection systems.
2. Description of the Related Art
Forward looking infrared (FLIR) based imaging systems are widely used to view targets and other objects in darkness and other conditions (smoke, haze, etc.) which might otherwise obscure visible detection. As it is often difficult to discriminate targets from background using an infrared imaging system, systems and methods have been developed for optimizing level and gain adjustments to enhance target detectability of IR imagers.
Many algorithms are known in the art for providing automatic gain and level control for infrared imaging systems. Unfortunately, these systems generally provide no manual adjustment capability. Those that provide manual level and gain control tend to change the picture drastically. This can cause confusion which, in certain (i.e. military) applications, can lead to serious adverse consequences. The dramatic change in the image is due to the fact that in automatic mode, output image intensity is related to detected energy in accordance with a nonlinear relationship using a histogram approach; whereas, in manual mode, output image intensity is related to detected energy in accordance with a linear relationship.
Hence, a need exists in the art for a system or method for improving the performance of infrared imaging displays. More particularly, a need remains in the art for a system or method for providing manual gain and level adjustment of the output of an infrared imager which does not substantially alter the image relative to that seen in an automatic gain and level adjustment mode thereof.
The need in the art is addressed by the system and method for providing manual level and gain control of an imaging system of the present invention. In accordance with the inventive method a plurality of signals are received from a detector representative of a scene. Next, a histogram of the scene is created. The histogram includes a number of intensity bins and an indication of the number of pixels in each. The histogram is transformed in accordance with a first function which defines how the histogram affects a display of the signals. The method allows for one to set a limit with respect to the first function. The method then determines a normalized cumulative distribution function of the transformed histogram within the limit on localized gain. Finally, the method is adapted to receive gain factor and level input from an operator and define a histogram controlled second function to assign values for each bin in accordance therewith and in accordance with the normalized cumulative distribution function.
In a specific implementation, the method further includes the steps of ascertaining a midscale gray bin for the level shifted transformed histogram and determining a clipped, level shifted normalized cumulative distribution function which maintains midscale gray of the midscale gray bin. In the illustrative embodiment, the step of receiving gain factor and level input further includes the step of determining a normalized cumulative distribution function for each bin. A gray shade is assigned for each bin in accordance with the second function. The inventive method further includes the steps of adjusting the polarity of each bin and effecting a gamma correction on each pixel in each bin.
In an illustrative application, the inventive system and method are used in the processing of digital video. The present invention allows for manual level and gain (MLG) adjustment of the video similar to that used in a histogram based automatic level and gain (ALG) control system using a cumulative distribution function (cdf) which is the area under the histogram. Level changes are made by an offset (L) to the existing ALG level (median) and gain changes are made by changing the relative gain multiplier (G) which acts on the ALG algorithm causing the gain to change equally about the median. ALG is thus a subset of the histogram based MLG in which G=1 and L=0. In accordance with the invention, as part of the histogram based MLG algorithm, the local gain is clipped to a predetermined maximum in order to prevent an overly noisy picture when too much gain is applied. The inventive method determines how to prevent a shift in the level when increasing the gain (i.e., G greater than 1) even when clipping is applied. An anti-saturation algorithm (histogram end bins are clipped) is also applied to prevent saturation from affecting the video too drastically. In order to provide the ability to change the picture from black to white an offset is required under low gain. Lastly, the inventive method allows for fine and coarse adjustments, and also includes histogram equalization (part of the first function), polarity reversal and gamma correction.